This invention relates to ink jet apparatus for ejecting droplets of ink, and more particularly, to ink jet apparatus utilizing hot melt ink and having a system for repriming after depriming caused by reversion of the ink to its solid state when the apparatus is unused.
The use of hot melt ink in ink jet systems, which ink is normally in a solid or frozen state but attains a liquid state or phase when its temperature is raised, has presented a number of advantages to ink jet apparatus. For a discussion of the characteristics of such ink and the uses thereof, reference is made to U.S. Pat. No. 4,390,369 and pending U.S. applications U.S. Ser. No. 610,627, filed May 16, 1984; Ser. No. 565,127, filed Dec. 23, 1983; and Ser. No. 644,542 filed Aug. 27, 1984, all assigned to the same assignee as this invention and incorporated herein by reference.
While the use of hot melt ink has presented advantages as discussed in the above references, it also creates an additional requirement for the design of the apparatus, including a special need for frequent repriming of the system. One of the characteristics of hot melt type ink is that it degenerates faster the longer it is maintained in its liquid phase, with the result that it is advantageous to permit cooling and freezing of the ink when the apparatus is not in use. However, a problem arises from the fact that hot melt ink contracts when it cools to a temperature below the melting point, which contraction of the ink results in depriming of the system. Thus, such hot melt ink apparatus has an increased need for an efficient priming/repriming system.